Penney Jr., U.S. Pat. No. 3,444,460 discloses a conductivity probe 10 which is mounted to measure the conductivity of matter such as fluids. A flat spiral coil 26 etched on a circular circuit board 42 is used as the probing element. (Col. 4, lns. 3-60, FIGS. 1-4). An etched shielding 32 is provided on the opposite side of the circuit board 42. (Id.) A coaxial conductor 44 surrounds the coil 26, board 42 and shielding 32. (Id.) The spiral coil 26 is connected by a conductor 50 to measuring circuitry in probe head assembly 14. (Col. 5, lns. 8-13, FIGS. 4-5) The conductivity probe 10 shown in Penney Jr., is adapted for mounting within a pipe structure 20, in which a conducting fluid is located. (Col. 4, lns. 3-18) The coil 26 forms part of a resonant circuit which is driven by a constant current at its resonant frequency. (Col. 5, lns. 13-22) In a vastly different application, Hirschi et al., U.S. Pat. No. 5,325,873, discloses an external probe 10 for detecting resonant circuits 32 and 34 in a feeding tube. (Col. 2, lns. 3-14, col. 4, lns.57-64.) Hirschi et al. specifically discusses a prior art system for detecting a metal object which was not suitable. The external probe 10 includes a circular search head 12 which further comprises a pair of parallel circular circuit boards 15 and 17 each of which contains a coil or coils. (FIGS. 1, 3, and 4) Coil 40, a circular transmitter coil and two receiver D shaped coils, 42a and 42b, are located on circuit board 17, while second circular transmitter coil 41 and two D shaped coils 44a and 44b are located on circuit board 15. (Id., col. 6, lns. 26-47) The coils are designed to produce a null condition when a target is centered. (See col. 6, lns. 48-55)
In the prior art, metal detector wands were injection molded in at least three sections, generally top, bottom and battery cover. This required higher molding costs, many screws to assemble the device and separation with overlap all around the device where the top meets the bottom. This overlap causes poor weather sealing. Metal detector wands are usually comprised of a handle portion and a coil portion. In the prior art a copper magnet wire or ferrite coil assembly was inserted into the coil portion of the metal detector wand. All or some of the detector circuitry was generally placed in the handle portion of the wand.
In the prior art shielding to reject body capacitance and other noise interference would often be added in the form of shielding tape, paint or conduit, often with a grounding wire or tab.